1. Field of the Invention
The present invention relates to an electrode for a nonaqueous electrolyte secondary battery, a nonaqueous electrolyte secondary battery, and a method of manufacture of the electrode for the nonaqueous electrolyte secondary battery.
2. Description of Related Art
A nonaqueous electrolyte secondary battery (e.g., a lithium ion secondary battery), as well known, has a positive electrode and a negative electrode. These electrodes have a structure in which an electrode material is attached on a current collector. The electrode material contains at least active material particles and electro-conductive material particles.
The electro-conductive material particles are used to improve electrical conductivity of the active material particles. Especially, at the positive electrode, to improve the electrical conductivity, the particle diameter of the electro-conductive material particle is equal to or smaller than the particle diameter of the active material diameter. In other words, if the particle diameter of the electro-conductive material particle is large, the electrical conductivity is insufficiently improved, and, usually, particles having a particle diameter larger than the active material particle diameter of the positive electrode are not used as the electro-conductive material.
As a typical example, lithium cobalt oxide is used as the active material of the positive electrode. In this case, acetylene black is chiefly used as the electro-conductive material. The lithium cobalt oxide is about 20 μm in average particle diameter, and the acetylene black is about 0.1 μm in average particle diameter. Besides, recently, from the viewpoint of safety and cost, iron lithium phosphate having a small particle diameter of about 15 to 0.05 μm is often used; in this case as well, an electro-conductive material having a particle diameter smaller than this is used.
Besides, natural graphite is often used as the active material of the negative electrode, and graphite as the electro-conductive material is added to the natural graphite. Generally, the natural graphite has a typical particle diameter of 20 to 30 μm, and the graphite has a typical particle diameter of 4 to 20 μm.
In the meantime, a lithium ion secondary battery has a large energy density and an excellent cycle characteristic. Therefore, the lithium ion secondary battery is used for power supplies of various apparatuses, is finding wider applications and is also expected as a power supply for household power.
However, in current circumstance, it is hard to obtain a large output and a large capacity is strongly desired. For example, for a large capacity of a secondary battery, it is proposed to use a high-molecular radical material as the active material of the electrode (e.g., JP-A-2010-114042).
This conventional technology has the following problems. Specifically, it is necessary to use a specific material (high-molecular radical material). Further, during a production time of the electrode, to prevent occurrence of a crack or a warp, it is necessary to add a special carbon fiber (carbon fiber that has an average fiber diameter of 0.01 to 0.5 μm, a fiber length of 15 to 100 μm, and substantially no branch structure) into the electrode.
Accordingly, it is preferable to achieve a large capacity of a nonaqueous electrolyte secondary battery without using the above specific material or special carbon fiber.
Further, it is preferable to achieve the large capacity by means of a method that has not been tried positively so far. Specifically, it is preferable to achieve the large capacity by increasing a coating amount per unit area of the electrode, that is, enlarging a thickness of the electrode material.
In the conventional nonaqueous electrolyte secondary battery, the electrode material has a thickness under 50 μm. However, it is possible to enlarge the capacity per unit area of the electrode in proportion to the thickness of the electrode material. For example, if the electrode material (e.g., electrode material of the positive electrode) has a thickness of 100 μm or larger, the capacity per unit area of the electrode also is two times or larger than conventional. Therefore, it is preferable that the electrode material (e.g., electrode material of the positive electrode) has a thickness of 50 μm or larger that is larger than conventional.
However, it is found out that there are the following problems if the thickness of the electrode material becomes large. Specifically, if the thickness of the electrode material becomes large, in a coating process, a drying process or after the drying during a production time of the electrode, a crack, which does not occur in the conventional, occurs on a surface of the electrode material attached on the positive electrode current collector, so that the electrode does not sufficiently function. Especially, in a case where the active material has a small particle diameter (e.g., nanoparticle), this phenomenon appears remarkably. Therefore, it is hard to use thick film coating.